Laser pulse train totalizer and interval counter

ABSTRACT

Pulses from a laser pulse train are detected and square wave shaped to trigger a first flip-flop in which one complementary output activating one input of a display comprising an array of light emitting diodes while the other output triggers a NOR gate and a second flip-flop, one complementary output thereof feeding a second input of the display with the other complementary output feeding the NOR gate which then feeds back to inhibit the first flip-flop. The square wave also activates a third flip-flop with one cpmplementary output thereof enabling a NOR gate for passing clock pulses which are decade divided and displayed on light emitting diode arrays. The other output of the third flip-flop feeds a pair of cross coupled NOR gates acting as a flip-flop, the output thereof being fed back to inhibit the third flipflop.

United States Patent Belz et al.

[451 March 6, 1973 LASER PULSE TRAIN TOTALIZER AND INTERVAL COUNTEROTHER PUBLICATIONS DAsaro et aL; Electronics; May 30, 1966; pp. 9 4-98.

Primary Examiner-Alfred E. Smith Attorney-Harry A. Herbert, Jr. et al.

[73 Assignee: The United States of America as 57 ABSTRACT represented bySecretary of the Air Force, Washington, DC Pulses from a laserpulse tramare detected and square I wave shaped to trigger a first flip-flop 1nWhlCh one Flledl 1971 complementary output activating one input of adisplay comprising an array of light emitting diodes while [21] A p].No; 207,754 the other output triggers a NOR gate and a second flip-flop,one complementary output thereof feeding a second input of the displaywith the other complemen- U-S. tary output feeding the gate which thenfeeds [5l nl- Cl G04f 9/ G04f 1 j 12 back to inhibit the firstflip-flop. The square wave also [58] Field of Search ..324/ I81, 186',250/21 1 J activates a third 'flip-flop with one cpmplernentary outputthereof enabling a NOR gate for passing clock pulses which are decadedivided and displayed on light [56] References Cited emitting diodearrays. The other output of the third flip-flop feeds a pair of crosscoupled NOR gates act- UNITED STATES PATENTS ing as a flip-flop, theoutput thereof being fed back to 3,656,060 4/1972 Bauernfeind et al...324/186 inhibi the bird 4 emit-15,3 Dawn riglires 7' V w H 55 i I 1-5g t 7mm 45' 41 I 'r 'l 6 MA I Am" ta; 1

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LASER PULSE TRAIN TOTALIZER AND INTERVAL COUNTER BACKGROUND OF THEINVENTION This invention relates to systems for analyzing pulsesequences, and more particularly to a device for counting pulses from alaser and determining the interval therebetween.

A double exposure hologram is usually taken by operating a ruby laser ina double pulse mode. Either a passive or an active Q-switch is used toproduce double pulses. In both cases the pulse separation at this timecan only be found by observing the pulses, or energy waveforms on anoscilloscope. When a passive Q- switch (a dye cell) is used, changes intemperature, dye concentration, and pumping energy can change the numberof pulses and their separation. In the event of multiple or singlepulsing, the laser parameters must be changed accordingly to obtain thedouble pulse condition.

In the past, the laser light was detected and the light intensity, orenergy, was displayed on a storage oscilloscope. This represents arather bulky and expensive piece of equipment, which is capable ofrecording the rapid pulse waveforms only as energy levels andnecessitates multiple firings to insure proper oscilloscope triggeringand signal presentation.

SUMMARY OF THE INVENTION The present invention is a system which can beused for the numerical display of the pertinent number of light pulses(0, one, two, or three) from a Q-switched laser and the time intervalbetween the first two. Rapid data acquisition and simplicity ofoperation make this invention unique in pulsed laser applicationsespecially when double pulsing is necessary for holographic velocimetrydata. An internal 100 MHz clock displays the time separation to inanoseconds. The system herein described performs a task that formerlyrequired the combined use of a storage oscilloscope, a laser photometerand a high speed frequency counter. The present invention represents anorder of magnitude decrease in equipment cost and possesses a degree ofportability and ease of operation that can not be equaled by themulti-instrument approach.

It is therefore an object of this invention to provide a novel andimproved pulse totalizer and interval counter.

It is another object to provide a pulse totalizer and interval counterfor analyzing pulses from a Q-switch laser particularly for doublepulsing needed for holographic velocimetric data.

It is still another object to provide a pulse totalizer and intervalcounter that eliminates the need for an oscilloscope for observing pulseseparation.

These and other objects, advantages and features of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the following drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial representation showingan embodiment of the invention;

FIG. 2 is a block diagram of an embodiment of the invention; and

FIG. 3 is a block diagram showing details of that shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. ll shows apictorial of the system wherein laser 1 1 produces the pulses which areto be counted and interval measured. The energy of the pulses arecarried by fiber optics bundle 13 to totalizer and interval counter 15although reflected light from an optical element can be detected also.The total number of light pulses are displayed by indicator 17 and theseparation between the first two pulses are displayed by indicator 19.This system is especially valuable for double pulsed velocitymeasurements although it is possible to obtain a general indication ofthe proper laser firing mode by the pulse number and indication. Morethan two pulses are indicated on numeric readout 17 as the number 3 andonly the time separation between the initial two pulses is displayed. Bythe use of blocking capacitors, the system is impervious to backgroundlight.

Counter 19 in the particular embodiment shown can count an interval upto 999.99 usec. If the count goes higher over range light 21 isactivated. Power to the system is controlled with switch 23 and thesystem is reset with switch 25.

The basic subsections of the pulse data readout are indicated in blockdiagram form in FIG. 2. The light from the laser is detected bydetector27 and the resulting electrical signal is amplified and shapedin circuits 29. These conditioned pulses drive the logic circuitry whichdisplays the numerals 0, l, 2, and 3, depending on the number of lightpulses detected. The first two pulses also open and close gate 35allowing signals from clock 37 to be encoded and the time between theopening and closing of gate 35 appear on display 39. Upon passage of thesecond pulse, the gate is inhibited from reopening until it is manuallyreset preventing the separation between other pulses from beingdisplayed. If only a single pulse occurs at the gate, it will remainopen until it is manually reset. In this case the numeric display of thepulse separation will remain at zero.

The detection of a 20 nanosecond duration laser light pulses andtheaccurate determination of the time interval between pulses requiresthe application of fast pulse techniques in electronic circuits whichare illustrated in FIG. 3. The packages which can be employed in thiscircuit are commercially available integrated circuits and for the mostpart are high speed emitter coupled logic types. Photo diode 41 with arise time t of less than 1 nanosecond senses the light pulse andintroduces a voltage pulse through coupling capacitor'43 into amplifier45 and then to Schmitt trigger 47 which shapes the input signal for thelogic circuits.

Flip-flops A and B (designated as 53), translators 55 and 57, NOR gate59, and the light emitting diode numeric indicator 61, comprise thepulse totalizer. This circuit counts each light pulse produced by thelaser up to three pulses and displays the results on numeric indicator61. At the third laser pulse A and B both appear at logic 0, i.e., atthe low level. NOR gate 59 has an output which is a logic l that is fedto the K input of flip-flop 49. The application of this logic stateinhibits further toggling of flip-flop 49 which in turn inhibitsflip-flop 53. These emitter coupled logic circuits are coupled totranslators 55 and 57 to permit voltage level translation from from theemitter coupled logic ECL circuits to transistor-transistor logic TTLwhich is accepted by light-emitting diode numeric indicator 61. Sinceonly four states need to be recognized, O to 3, a two-line coded inputto numeric indicator 61 is sufficient. This indicator can be a 5 X 7matrix or lightemitting diodes containing internal current drivers and abinary coded decimal decoder.

Velocity measurement requires accurate data for the determination of At,which is the time interval between 2,, the leading edge of the firstpulse and t the leading edge of the second pulse. The first two pulsesof the input pulse train cause flip-flop 51 to produce a gating pulse toclock gate 64. During the gating pulse interval clock pulses from clock63, in the example a precision 100 MHz crystal clock oscillator, areallowed to pass through driver 64 and into a series of flip-flops 65-68which serve as the first decade divider. At the terminal edge of thegating pulse resulting from the second light pulse, numeric indicators75-79 display the elapsed time between t and t resolved to 10nanoseconds. Indicator 75 displays the least significant digit. Themaximum time interval that may be recorded is 999.99 microseconds.

Flip-flop 51 initially is preset with the C output at logic 1 whichchanges to logic 0 at the first laser pulse. The complement terminal 6likewise goes to l causing NOR gates 81 and 83 which are a cross-coupledpair serving as a set-reset flip-flop to return a logic I to the K inputof flip-flop 51. At the next clock pulse into flip-flop 51 the Cterminal will return to logic l and no further change in the flip-flopstate will occur, i.e., flip-flop 51 is inhibited by the shifted setapplied to the K terminal.

If the C terminal of flip-flop 51 is l, gate 64 is closed to the pulsesfrom clock 63. Thus gate 64 serves as the countermain gate. Special highspeed flip-flops are required to count the 100 MHz clock pulse train.The output frequency of the pulse train from the divider is 10 MHZ thuspermitting further divider packages 71-74 to be of the slow speed TTLtype. If over-range occurs divider 71 will activate over-range indicator70. The commercially available decade divider available as a single 14pin dual-in-line package can be interfaced directly with the numericindicator. Data transfer is made by a four line binary coded decimalcircuit.

The emitter coupled logic divider 65-68 requires voltage translators83-86 between indicator 75. A further voltage translator 81 is requiredof the 10 MHz pulse train entering the transistor-transistor logicdivider 74. At the completion of the second laser pulse numericindicators -79 will display the elapsed time interval t t The numericindicators are reset to read 0 for subsequent laser firings bydepressing the reset button 89. NOR gates 91 and 93 together act as anR-S flip-flop. The increase in voltage at NOR gate 93 changes the stateof the output of this R-S flip-flop configuration to a logic level lwhich resets all flip-flops and decade dividers to the desired initialconditions. The transistor 95 interfaces this logic state with all thereset terminals along the TTL decade divers 71-74.

It is claimed:

. A system for simultaneously counting laser pulses and determining theinterval therebetween comprising:

a. a first totalizer flip-flop fed by the pulses and having first andsecond complementary outputs;

b. a second totalizer flip-flop fed by the first complementary output ofthe first totalizer flip-flop having first and second complementaryoutputs;

c. a totalizer NOR gate fed by the first complementary output of thefirst totalizer flip-flop and the first complementary output of thesecond totalizer flipflop, the output of the totalizer NOR gate beingfed to the input of the first flip-flop;

d. a totalizer diode array display fed by the second complementaryoutput of the first totalizer flipflop and the second complementaryoutput of the second totalizer flip-flop;

e. a first counter flip-flop fed by the pulses and having first andsecond complementary outputs;

f. a clock;

g. a NOR gate fed by the clock and enabled by the first complementaryoutput of the first counter flip-flop;

h. a second counter flip-flop fed by the second complementary output ofthe first counter flip-flop and feeding the input of the first counterflip-flop;

i. a decade divider having a sequence of stages fed by the NOR gate; and

j. a diode display fed by the decade divider.

2. A laser counting and interval measuring system according to claim 1which further comprises an over range indicator fed by the output of thelast stage in the sequence of the decade divider.

3. A laser counting and interval measuring system according to claim 2which further comprises means for resetting the decade divider.

4. A laser counting and interval measuring system according to claim 3which further comprises:

a. a diode light detector for sensing the laser pulses;

and

b. a Schmitt trigger interposed between the diode light detector and thefirst totalizer flip-flop.

1. A system for simultaneously counting laser pulses and determining theinterval therebetween comprising: a. a first totalizer flip-flop fed bythe pulses and having first and second complementary outputs; b. asecond totalizer flip-flop fed by the first complementary output of thefirst totalizer flip-flop having first and second complementary outputs;c. a totalizer NOR gate fed by the first complementary output of thefirst totalizer flip-flop and the first Complementary output of thesecond totalizer flip-flop, the output of the totalizer NOR gate beingfed to the input of the first flip-flop; d. a totalizer diode arraydisplay fed by the second complementary output of the first totalizerflip-flop and the second complementary output of the second totalizerflip-flop; e. a first counter flip-flop fed by the pulses and havingfirst and second complementary outputs; f. a clock; g. a NOR gate fed bythe clock and enabled by the first complementary output of the firstcounter flip-flop; h. a second counter flip-flop fed by the secondcomplementary output of the first counter flip-flop and feeding theinput of the first counter flip-flop; i. a decade divider having asequence of stages fed by the NOR gate; and j. a diode display fed bythe decade divider.
 1. A system for simultaneously counting laser pulsesand determining the interval therebetween comprising: a. a firsttotalizer flip-flop fed by the pulses and having first and secondcomplementary outputs; b. a second totalizer flip-flop fed by the firstcomplementary output of the first totalizer flip-flop having first andsecond complementary outputs; c. a totalizer NOR gate fed by the firstcomplementary output of the first totalizer flip-flop and the firstComplementary output of the second totalizer flip-flop, the output ofthe totalizer NOR gate being fed to the input of the first flipflop; d.a totalizer diode array display fed by the second complementary outputof the first totalizer flip-flop and the second complementary output ofthe second totalizer flip-flop; e. a first counter flip-flop fed by thepulses and having first and second complementary outputs; f. a clock; g.a NOR gate fed by the clock and enabled by the first complementaryoutput of the first counter flip-flop; h. a second counter flip-flop fedby the second complementary output of the first counter flip-flop andfeeding the input of the first counter flip-flop; i. a decade dividerhaving a sequence of stages fed by the NOR gate; and j. a diode displayfed by the decade divider.
 2. A laser counting and interval measuringsystem according to claim 1 which further comprises an over rangeindicator fed by the output of the last stage in the sequence of thedecade divider.
 3. A laser counting and interval measuring systemaccording to claim 2 which further comprises means for resetting thedecade divider.